EFFECT OF CHEMICAL ADMIXTURES ON HYDRATION AND STRENGTH OF FINE-GRAINED CONCRETE

Abstract

Abstract. The performance of fine-grained concrete strongly depends on chemical admixtures, yet the combined effect of superplasticizers and air-entraining agents on early hydration and strength remains insufficiently understood. This study evaluates the influence of a polycarboxylate-ether superplasticizer (PS) and an air-entraining admixture (AER), applied separately and in combination, on the hydration kinetics, compressive strength, and microstructure of fine-grained concrete. PS (0.5–1.5 % by mass of cement) and AER (0.10–0.30 %) and their combinations were tested against a control paste at W/C = 0.28. Early-age hydration was assessed by temperature-evolution monitoring, determining the maximum hydration temperature (T_max) and the time to peak (t_max); compressive strength was measured at 7 and 28 days, and the microstructure was examined by SEM. AER progressively reduced T_max (by up to 17.8 %) and delayed hydration (t_max up to +52.7 %). PS at an optimal dosage of 1.0 % produced the highest T_max (87.5 °C), while combined PS–AER systems showed the strongest retardation (t_max up to 18.89 h). PS increased compressive strength by up to 33.2 % at 7 days, whereas AER alone markedly reduced it. The optimal combination SA0.1 (1.0 % PS + 0.10 % AER) reached 27.3 MPa at 7 days and 34.0 MPa at 28 days. SEM revealed a denser C–S–H network for PS-modified samples and characteristic spherical air voids in AER-modified ones. Overall, a superplasticizer at 1.0 % combined with a low AER dosage (0.10 %) optimally balances hydration, strength, and microstructure.